High performance sailboats are usually provided with a mainsheet system which is connected to a traveler mounted on the aft deck or on a bridge in the cockpit or on the cabin trunk. The traveler includes a car which rolls along a track. Lines are connected to the car to control its travel and position along the track. In light air the boat will frequently sail better if the car is trimmed to windward of the centerline of the boat. In heavy air the car may be let out considerably to leeward to spill air from the mainsail.
Assuming the car is trimmed to windward, the traveler control line is cleated to hold the car in the desired windward position. When the boat is to be tacked, the car has to be released so that it can travel to the other side of the boat to thereby enable the car to be trimmed to windward after the a tack has been completed. If the control line isn't released the car can't get over to the desired new position.
When tacking a boat things can get pretty busy at the helmsman station and this frequently results in the car ending up in a leeward position at the end of the tack. This is the very time when power and pointing ability are necessary to regain speed and the car should be trimmed to windward. If the helmsman is too concerned with trimming the car to windward, he probably won't be steering the boat very well and in competitive racing this is a serious concern.
There have been efforts in the industry to provide some means of retaining the car in a windward position at the completion of a tack. The solutions thus far offered are not practical and have been withdrawn from the market.